1. Field of the Invention
The present invention relates to a display device, and more particularly to a liquid crystal display device comprising a novel liquid crystal microstructure formed by a method controlling the microstructure, and a method for forming a novel colloidal liquid crystal composite (CLCC). The present invention also relates to a novel electro-optical device using the novel colloidal liquid crystal composite and a method for forming the electro-optical device.
2. Description of the Related Art
Recently, liquid crystals have been used in various applications and their role in the display field has become more and more important. The applications of the liquid crystals include, for example, computer display screens, wristwatches, architectural windows, privacy windows, automobile windows, automobile sunroofs, switching devices for optical systems, projection display devices, reflective display devices, hand-held paging devices, cellular phones, laptop computers, television screens including car-mounted television screens, automotive displays including radio, dashboard, and on-board navigation systems, helmet displays such as xe2x80x9cheads-upxe2x80x9d displays, cockpit displays, imaging devices, virtual reality devices, simulation devices, electronic gating devices, diffraction gratings, and calculators. The liquid crystal devices used in the above applications may be monochromatic or polychromatic.
Liquid crystal molecules have generally a rod-like or a disk-like shape, and their physical properties are intermediate those of a crystalline solid and those of an amorphous liquid. The liquid crystal molecules comprise, generally, substituted biphenyl or triphenyl functional groups, in which one of the phenyl groups is separated by a spacer group. Liquid crystal types include, for example, twisted nematic, super twisted nematic, cholesteric, ferroelectric liquid crystals. Liquid crystal devices (LCD) may be assembled by any of the liquid crystal types described above.
Alternatively, another type of display utilizing a liquid crystal dispersed within a polymer matrix in which the liquid crystals create micro-domains (i.e., a polymer dispersed liquid crystal display (PDLC) is known in the art. PDLCs are often used in the form of thin films less than about 200 xcexcm in thickness, and typically from 2 xcexcm to 50 xcexcm. It is known that light transmittance of PDLCs varies with reflective indices of materials and the angle of the reflection varies with respect to both the wavelength of light and temperature.
Although liquid crystals exhibit excellent availability as a display device and an electro-optical device as described above, developments for the liquid crystals are still continued so as to improve their physical properties for overcoming some deficiencies. The conventional liquid crystal devices have a narrow viewing angle. In addition, conventional PDLCs have undesirably high switching voltage resulting in higher driving power, and require additional apparatus for production thereof, which incurs an additional production cost and capital investment.
Several conventional types of display devices and the electro-optical devices have been proposed in relation to liquid crystals having dual-domain structures.
It is known that a viewing angle of the twisted nematic display device is asymmetric, and its contrast becomes poor when viewed at certain angles. Many attempts to overcome the above problem have been made by creating a dual-domain structure in the liquid crystal, in which the liquid crystal molecules are aligned with different directions in adjacent domains at each display pixel. The above structure has been realized by various techniques, all of which disadvantageously require an additional process to form two alignment surfaces.
FIG. 1 depicts a schematic cell structure having the above architecture and a schematic production process for obtaining the above structure. The structure shown in FIG. 1 is a dual-domain twisted nematic device whose pixel is made up of two subpixels. As shown in FIG. 1, the simplest way to obtain the above structure is by first rubbing alignment layers coated onto the substrate, patterning photolithographically such portions that need to be masked, rubbing the unmasked portion in a different direction, and then removing the mask pattern. Recently, a special photo-alignment material has become available and the alignment may be controlled by the polarization of irradiated light and any pattern can be created without the masking process, when the light is patterned (E. Hoffman et al. SID""98 Digest, p. 734, 1998).
The top and bottom substrates are positioned at a right-angled alignment, and the two subpixels have different tilt angles. The subpixels, therefore, have 90 degree twist structures with right and left handednesses. Due to the symmetry of the two twist structures, the pixel exhibits a symmetric viewing angle at any bias level, and hence the anisotropy of the viewing angle may be reduced.
However, the cost and time required for obtaining the dual domain structure described above by using these techniques hinder versatile commercialization.
In addition to nematic liquid crystal technologies, there are other devices that take advantage of optical properties of cholesteric liquid crystals. Cholesteric liquid crystals show two states when sandwiched between two substrates (i.e. a planar state that exhibits reflection over a narrow band of wavelengths and a focal conic state which scatters light weakly). These two states are shown in FIGS. 2(a) and 2(b). Bi-stable director distributions respectively correspond to the planar state in FIG. 2(a) and a focal conic state in FIG. 2(b) in the cholesteric device. The bi-stable operation between these two states must be secured by either a polymer network formed in the liquid crystal or surface treatments. The former is called a xe2x80x9cpolymer-stabilized cholesteric texturexe2x80x9d (D. K. Yang et. al., xe2x80x9cControl of reflectivity and bistability in displays using cholesteric liquid crystalsxe2x80x9d, J. Appl. Phys. Vol. 76, No. 2, pp.1331-1333) and devices based on these systems are known and marketed by Kent Display.
However, a drawback of this technique is that an additional UV curing process is required to obtain a polymer network. Recently a new kind of the cholesteric texture modes has been presented (U.S. Pat. No. 5,956,113 to G. P. Crawford et. al. incorporated herein by reference) in which a small amount of commercial inorganic silica particles is blended in a host cholesteric liquid crystal, and the aggregates of the particles due to the hydrogen bond develop network structure in the host.
The presence of molecular interaction between the particles and liquid crystal molecules may contribute to promote bi-stability of the device. This technique is prominent in its simple processabilty. However, there is no visible cellular morphology nor remark to the mechanical properties of the liquid crystal composites, or to whether such compositions are allowed to be poured and processed as typical liquid crystal materials. The low viscosity of the composite will limit processing conditions under which the layer of liquid crystal composites may be formed on the substrate.
This device utilizes a small amount of dye compound dissolved in a host liquid crystal material and provides a reflective display (White and Taylor, J. Appl. Phys., Vol. 45, p. 4718, 1974). A typical construction of a White-Taylor device is shown in FIGS. 3(a) and 3(b). Usually, a White-Taylor device has a 300 degree twist configuration provided by a polyimide coating and a rubbing processes. In FIG. 3(a), light is absorbed in a bias-off state by the liquid crystals, whereas the incident light passes through in a bias-on the state in FIG. 3(b). The curved arrows in FIG. 3(a) indicate the helical structure of the molecules in the bias-off state. The device appears dark under the bias-off state because the molecular axes of dye molecules, which lie along the substrate surface, rotate almost a full turn, thereby maximizing the light absorption as shown in FIG. 3(a). When the bias voltage is applied to the device as shown in FIG. 3(b), the dye molecules are forced to align along with the electric field, thereby minimizing the light absorption of the molecules.
The voltage vs. transmittance curve of a typical White-Taylor device is shown in FIG. 4. The solid line shows the transmittance when a bias voltage is decreased and the dashed line shows the transmittance when the bias voltage is increased. The voltage vs. transmittance curve shown in FIG. 4 exhibits a sharp rise at about 2.5 V while the hysterisis on the response is observed. At this voltage, stripe domains start to grow and spread over the entire region for a few minutes. It is considered that the formation of the stripe domains also causes the hysteresis on the voltage vs. transmittance curves.
This device also works as a bi-stable device having substantial hysteresis. This problem may be overcome by using a specific alignment treatment called a-N*GH (T. Sugiyama et. al., xe2x80x9cA reflective a-N*GH-LCD and its ergonomic characterizationxe2x80x9d, SID""96 Digest, pp. 35-38, 1996), where a director angle is distributed randomly in the micro-domains. A cell is prepared by cooling down, after filling in a liquid crystal at a very slow rate. Therefore, the process requires a long processing time and precise temperature control, thereby incurring extra cost in the production process.
U.S. Pat. No. 5,976,405, incorporated herein by reference, discloses uniformly sized domains of liquid crystals surrounded by a polymer shell, which is also known as polymer-encased liquid crystals (PELCs). A display made using the PELCs exhibits markedly improved electro-optical performance. However, the cost and time required for obtaining the dual domain structure by using these techniques are also obstacles for versatile commercialization of the devices.
Therefore, a novel liquid crystal material having wide viewing angle and low switching voltage has been desired. In addition, a novel liquid crystal material, which allows a display device to be produced without incurring an additional production cost, has been requested.
Additionally, a novel liquid crystal material, which allows an opto-electric hysterisis to be controlled, has been desired. Further, a novel display device including such novel liquid crystal material and a novel electro-optical device (and a method for forming the same) have been desired. Hitherto the present invention, such desires and requests have not been met.
In view of the foregoing and other problems, disadvantages, and drawbacks of the conventional methods and structures, an object of the present invention is to provide a method and structure using a novel liquid crystal material.
Another object of the present invention is to provide a transmission or reflection type liquid crystal display device having controlled polydomain microstructures.
Another object of the present invention is to provide a liquid crystal display device for reducing the angle-dependence of the display""s viewing angle for major device types including at least twisted-nematic, super-twisted-nematic, parallel nematic, cholesteric and White-Taylor architectures.
Yet another object of the present invention is to provide a liquid crystal display device which does not require further processing steps in device fabrication processes.
Still another object of the present invention is to provide a liquid crystal display device which allows hysteresis in electro-optical switching characteristics to be controlled.
Still another object of the present invention is to provide an electro-optical device comprising the above colloidal liquid crystal composite and a method for forming the same.
In a first aspect of the present invention, a display device includes substrates facing each other and each having an electrode, the substrates being spaced apart, spacers disposed between the substrates and defining an electro-optical cell together with the substrates, and a colloidal liquid crystal composite (CLCC) provided in the electro-optical cell, the CLCC comprising particle-rich inter-domain regions and micro-domains of a liquid crystal, the particle-rich inter-domain regions comprising colloidal organic particles being networked in the liquid crystal.
In a second aspect, a method of forming a liquid crystal microstructure according to the present invention includes providing colloidal organic particles, and mixing the colloidal organic particles with a liquid crystal so as to form micro-domains under a condition that the colloidal particles are introduced in an isotropic phase of the liquid crystal.
The inventors completed the present invention through work on CLCCs which provide advantages on the complete range of physical and optical properties of liquid crystals. The basic idea of the invention is widely applicable to controlling liquid crystal micro-structures in order to improve the optical performance of display devices and electro-optical devices.
A general principle of the present invention is that the naturally-occuring internal interfaces of the CLCC microstructure provide domain boundaries of controlled size depending on colloidal particle concentration. Moreover, a formation of the cellular microstructure may arise from partial phase separation of the particles driven by emerging nematic domains. These domains expel the colloidal particles for lowering an elastic distortion energy when passing through the isotropic-nematic transition point.
The present invention uses a network created by colloidal organic particles with a surface treatment which stabilizes and controls microstructures in liquid crystal (LC) materials, thereby improving optical and physical performances of the liquid crystal display device and the electro-optical device including the liquid crystals. The resulting colloidal liquid crystal composite is stabilized by the colloidal organic particles made from polymeric materials. Surfaces of the organic colloidal particles may be grafted with poly-12-hydroxystearic acid so as to sterically stabilize the colloidal particles. Incorporation of the colloidal particles into the liquid crystal may be achieved by heating the liquid crystal above its clearing points (i.e., clearing temperature) so as to ensure mixing between the organic colloidal particles and the liquid crystal over a cell region. Also, the present invention provides a method for forming a microstructure suitably utilized for a liquid crystal display.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-126453, filed Apr. 26, 2000, which is expressly incorporated herein by reference in its entirety.